Linear alpha-olefins are widely used commercially as important materials used for comonomers, detergents, lubricants, plasticizers, etc., and in particular, 1-hexene and 1-octene are widely used as comonomers for controlling the density of polyethylene during the preparation of linear low-density polyethylene (LLDPE).
In existing preparation processes of linear low-density polyethylene (LLDPE), ethylene is copolymerized with alpha-olefins, for example, comonomers such as 1-hexene and 1-octene, so to control the density thereof by forming branches in a polymer backbone.
Accordingly, for the preparation of LLDPPE with a high content of copolymers, there was a problem in that the cost of comonomers occupies a large part of preparation costs. There have been various attempts to solve these problems.
In addition, since alpha-olefins have various different application fields or market sizes according to the kind, a technology of selectively producing a specific olefin is commercially very important, and recently, many studies are being progressed on the chromium catalyst technology for preparing 1-hexene or 1-octene with high selectivity through selective ethylene oligomerization.
Existing commercial preparation methods for preparing 1-hexene or 1-octene include the SHOP process of Shell Chemical, the Ziegler process of Chevron Philips, and the like, whereby C4-C20 alpha-olefins with a wide distribution can be produced.
As a catalyst for trimerization of ethylene, a chromium-based catalyst using a ligand of the general formula (R1)(R2)X-Y-X(R3)(R4) has been proposed. In the formula above, X is phosphorus, arsenic, or antimony, Y is a linking group such as —N(R5)-, and at least one of R1, R2, R3, and R4 has a polar or electron-donating substituent.
Additionally, as a ligand that exhibits catalytic activity to 1-hexene under catalytic conditions, studies have been progressed on o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2, which does not have a polar substituent on at least one of R1, R2, R3 and R4 (Chem. Commun., 2002, 858).
However, with regard to ligands containing heteroatoms of the above-mentioned prior art, there has been a continuing demand for multimerization reaction activity and high selectivity which are consistently maintained during the reaction when preparing 1-octene or 1-hexene.
In view of the above, the inventors of the present invention conducted intensive studies on ligands capable of solving the above-mentioned problems, and as a result, found that a trans type ligand is suitable for olefin oligomerization as described later, thereby completing the present invention.